1. Field of the Invention
The present invention relates to a flow rate control valve, which is capable of controlling the flow rate of a fluid that flows through a flow passage by displacing a valve plug in an axial direction under the driving action of a driving source.
2. Description of the Related Art
The following structure has hitherto been adopted for a flow rate control valve. Specifically, an inner cylinder, having a screw section formed on the inner circumferential surface thereof, is rotatably provided within a valve housing. A valve shaft, having a valve plug fixed at one end thereof and a screw section disposed on its outer circumferential surface, is inserted into the inner cylinder. Screw sections of the inner cylinder and the valve shaft are threadedly engaged with each other.
In this arrangement, the inner cylinder is rotated by means of a stepping motor, while inhibiting rotation of the valve shaft, in a state in which the screw sections of the inner cylinder and the valve shaft are threadedly engaged with each other. The valve shaft is moved in the axial direction, thereby controlling opening and closing of the valve plug, so as to control the flow rate of the fluid. An example of the above-described conventional technique is disclosed, for example, in Japanese Laid-Open Utility Model Publication No. 57-54636.
In the case of the flow rate control valve described above, a lubricant such as grease is applied beforehand to the screw sections of the inner cylinder and the valve shaft, in order to avoid abrasion of the screw sections. However, for example, when pure water flows as the fluid through the valve, since the screw section exists inside the inner cylinder, there is a disadvantage in that such pure water may become polluted by the lubricant.
When the liquid, such as pure water, flows into the valve chamber of the flow rate control valve, high water pressure is exerted in one direction of the valve plug. As a result, the water pressure is applied in one direction of the screw section of the valve shaft, and frictional force is increased between the screw section of the valve shaft and the screw section of the inner cylinder. Consequently, a high rotational torque is required to rotate the inner cylinder, and abrasion is likely to occur at the screw sections of the valve shaft and the inner cylinder. Therefore, the service life of the flow rate control valve may be shortened.